1. Field of the Invention
The present invention relates to imaging. More specifically, it relates to imaging devices that have phosphor layers showing high resolution and high luminance and being typically suitable for projection cathode-ray tubes.
2. Description of the Related Art
Imaging devices discussed herein are those for imaging based on image data through light emission in such a way that phosphors are excited by irradiation of electron beams or ultraviolet rays. Examples thereof are cathode-ray tubes (especially, projection cathode-ray tubes), display panels using low energy electron beams, such as field-emitter displays (FEDs), and plasma display panels (PDPs). The field-emitter displays herein are displays having electron emitters arranged as a matrix and include those typically using, as the electron emitters, surface condition electron emitters (SCEs), or thin-film electron emitters such as metal-insulator-metal (MIM) electron emitters.
The imaging devices also include a system for imaging from image data which incorporates, for example, a drive for driving any tube or panel mentioned above, and image data processing circuitry. They further include non-self-luminous imaging devices each including a non-luminous display unit such as a liquid crystal display, and a light source as a backlight or sidelight, in addition to self-luminous imaging devices as above.
Higher resolution of these imaging devices has been achieved to-meet the need of higher performance. For example, higher resolution of imaging devices of electron beam excitation systems is achieved by reducing the diameter of an excitation spot of electron beams, increasing the scan speed, and increasing the excitation intensity. However, this causes luminance saturation of phosphors used in the imaging devices, luminance degradation, and significant afterimages due to afterglow, resulting in a decline in image quality. In addition, better color reproduction quality in light emission is required. Therefore, phosphors must satisfy requirements in luminance and luminance degradation properties, afterglow properties and color enhancement.
Taking a projection-type cathode-ray tubes (hereinafter referred to as a projection tube) typical of the imaging devices as an example, its problems will be explained below. The cathode-ray tube controls its luminance intensity by regulating the current of excited electron beams. Thus, it is required that the luminance of phosphors linearly increases in proportion to the current. However, generally, as the excitation intensity becomes high, luminance saturation takes place, that is, the luminance runs off the linear. When an image is displayed with highly intense excitation, phosphor materials are damaged. Consequently, luminance decreases and color emission degrades during continued use of the projection tube.
The projection tube is a cathode-ray tube for use in a projection-type display and projects an image generated by the cathode-ray tube on a screen through the optics that enlarge the image area by several tens of times. Thus, excitation is performed by current of 10 to 100 times as much as a generally used direct-viewing cathode-ray tube producing non-enlarged images. Accordingly, requirements for phosphors for the projection tube are less luminance saturation especially when the tube carries a large quantity of current and less degradation when the tube carries a large quantity of current.
Among the phosphors, especially for green emitting phosphors which generate 70% of luminance in a white light, the above improvements of phosphor properties are important. Varieties of materials have so far been used for green emitting phosphors for the cathode-ray tube. An example of such phosphors is a phosphor composition expressed by chemical formula Y2SiO5:Tb. The feature of this phosphor composition is less luminance saturation when excited with high-density current and it has been generally used as a practical phosphor.
To improve the properties of this phosphor, attempts have been made to improve luminance by adding a scandium (Sc) oxide to materials and firing the mixture, as disclosed, for example, in Japanese Patent Publication No. 61-21505 and Japanese Patent Publication No. 06-62939. Improvements to luminance and suppressing luminance degradation were attempted by adding at least one of Gd, Tm, Sm, and Eu to materials and firing the mixture, as disclosed in, for example, Japanese Laid-Open No. 02-289679.
Luminance improvement was attempted by replacing a part of the composition with Mn, as disclosed in, for example, the above-mentioned Japanese Patent Publication No. 61-21505. Luminance improvement was attempted by replacing a part of the composition with Dy or Pr, as disclosed in, for example, Japanese Patent Publication No. 06-60354. Luminance degradation improvement was further attempted by using an excess of SiO2 as disclosed in Japanese Laid-Open No. 2003-115481.
The conventional phosphor layers containing conventional phosphors have a thickness exceeding 40 μm so as to improve their luminance. However, the emitted light has a larger diameter (spot diameter), because the emitted light extends in the such thick phosphor layers. The resulting phosphor layers thereby fail to provide sufficient resolution. Even the above-mentioned improved phosphors show insufficient luminance that fails to meet recent requirements of higher performance. The insufficient luminance requires the application of electron beams at high current density, thereby causes coloring and decreased light emission efficiency, and invites luminance which decreases with elapse of irradiation time.